Process of producing esters of phthalic acids by oxidation of xylene and toluic acid ester mixtures



PROCESS OF PRODUCING .ESTERS F PHTHALIC ACIDS BY OXIDATION OF XYLENE ANDTOLU- IC ACID ESTER MIXTS No Drawing. Application June 28, 1957 SerialNo. 668,606

Claims priority, application Germany July 2, 1956 12 Claims. (Cl.260-475) The present invention relates to the production of esters ofphthalic acids and more particularly to the production of esters ofphthalic acids from mixtures of xylenes and esters of toluic acids withmonovalent alkanols having 1 to 4 carbon atoms by oxidation by means ofoxygen-containing gases at elevated temperature.

It is one object of the present invention to provide a simple and highlyadvantageous process of producing esters of phthalic acids by oxidationof mixtures of xylenes in mixture with esters of toluic acids wherebysubstantially all of the xylene starting material is converted intooxidation products and whereby the formation of undesirable by-productsis considerably reduced.

Other objects of the present invention and advantageous features thereofwill become apparent as the description proceeds.

The process according to the present invention consists in using asstarting material a mixture of xylenes and esters of toluic acids withalkanols having 1 to 4 carbon atoms in the proportion by weight betweenabout 1:3 and, at the most, 1:1. Said mixture is oxidized by means ofoxygen or oxygen-containing gases, such as air, at a temperature betweenabout 80 C. and about 250 C. and preferably between about 140 C. andabout 160 C. at ordinary or increased pressure for a prolonged period oftime, at least for about four hours. Thereby first all the xylenes areconverted into toluic acids. At the same time the toluic acid esterspresent in the starting mixture are, at least partly, oxidized tophthalic acid monoesters. The resulting toluic acids sufierintermediarily an ester radical interchange reaction with the phthalicacid monoesters formed on said oxidation so that toluic acid esters areproduced. The resulting mixture of toluic acid esters and phthalic acidsis then further oxidized until as much toluic acid ester is oxidized ascorresponds to the toluic acids which were obtained by the initialxylene oxidation. In this manner a degree of oxidation is achieved whichpermits to produce, after subsequent esterification of the resultingoxidation mixture, 40% to 60% of diesters of phthalic acids, calculatedfor the total amount of oxidation products.

As stated above, the oxidation is carried out at elevated temperaturebetween about 80 C. and about 250 C. and preferably between about 140 C.and about 160 C. It is possible to operate at atmospheric pressure.Preferably, however, oxidation is effected under pressure. A pressurebetween about 1.5 atm. and about 5.0 atm.. has proved to be sufficientin order to obtain satisfactory results. A higher pressure may, ofcourse, also be used although it requires more complicated apparatus anda careful supervision of the reaction. Oxidation is preferably effectedin the presence of catalysts. Especially suitable catalysts arecompounds of multivalent metals, i.e. of metals which, like cobalt ormanganese, occur in several stages of valency. Such metal compoundsatcnt O must be soluble in the reaction mixture. Especially suitableare, for instance, salts of cobalt with fatty acids having 6 to 12carbon atoms, such as the first runnings of the distillation of coconutoil and the like fatty acids or cobalt salts with aromatic carboxylicacids, such as toluic acids or benzoic acid.

The process according to the present invention can be carried out inbatches as well as in continuous operation. When carrying out theprocess in continuous operation, it is advisable to effect oxidation ina number of apparatus units in order to ensure proper proceeding of thereaction. In continuous operation care must be taken that initiallyoxidation is carried out in a xylene. contalmng phase and thatsubsequently the intermediate oxidation mixture is further oxidized in axylene-free phase.

One of the advantages of the simultaneous oxidation of xylenes andtoluic acid esters according to the present invention consists inyielding a mixture of oxidation products at the end of the oxidationwhich mixture contains unreacted toluic acid esters of a considerablyhigher dissolving power for the oxidation products than possessed by thexylenes.

Another advantage of the simultaneous oxidation of xylenes with anexcess of toluic acid esters according to the present invention consistsin the decrease in the formation of by-products which are usuallyobtained in oxidation of xylenes.

Still another advantage of the process according to the presentinvention consists in a considerable acceleration of the reaction.

In principle, the oxidation of p-xylene to p-toluic acid in the presenceof the methyl ester of p-toluic acid is carried out as follows:

When operating in batches, a solution of p-xylene in the methyl ester ofp-toluic acid, for instance, a solution containing 35 parts by weight ofxylene and 65 parts by Weight of the ester, is subjected to oxidation bymeans of air in the presence of a cobalt compound. The gases escapingfrom the reaction vessel are cooled and the condensate obtained therebyis separated from the water present therein and is returned to thereaction vessel. When operating at a temperature between about C. andabout C. under slightly increased pressure, for instance, under apressure between about 1.5 atm. and about 3.0 atm., initially oxidationof p-xylene proceeds slightly more rapidly than that of the methyl esterof p-toluic acid. However, as soon as the xylene concentration has beenreduced considerably, vigorous oxidation of the methyl ester of p-toluicacid sets in. Oxidation is continued until an amount of the methyl esterof p-toluic acid has been oxidized to the monomethyl ester ofterephthalic acid which corresponds substantially to the molal amount ofp-toluic acid that has been formed by oxidation of p-xylene. Said amountcan readily be determined by analysis of samples taken from time totime. The resulting oxidation mixture which is free of xylene is thenesterified with methanol, the dimethyl ester of terephthalic acid formedthereby which is insoluble in methanol is filtered ofi, excess methanolis removed, and the non-oxidized methyl ester of p-toluic acid, theamount of which corresponds approximately to that charged initially, isagain subjected to oxidation after addition of p-xylene in an amountsufficient to maintain the xylene-p-toluic acid equilibrium, requiredaccording to the present invention.

When carrying out the process acccording to the pres ent invention incontinuous operation, oxidation is effected according to thecountercurrent principle in a reaction tower or in a number of oxidationvessels.

An essential characteristic of the process according to the presentinvention is the specific proportion of xylene Patented July 14, 1959 totoluic acid ester as well as the considerably prolonged reactionduration. Said reaction duration is dependent upon the reactiontemperature, the pressure, the amounts of starting mixture subjected tooxidation, and the amount of oxidation agent introduced. It should notbe less than about 4 hours. As stated above, the proportion of xylene totoluio acid ester must be maintained between about 1:3 and, at the most,1:1 expressed in proportions by weight. With other proportions there isno assurance that the above mentioned yield of phthalic acid ester isobtained at the end of the oxidation procedure.

It is furthermore very important that at the end of the oxidationprocess considerable amounts of non reacted toluic acid ester arepresent in the oxidation mixture. Said toluio acid esters serve assolvents or, respectively, distributing agents for the oxidationproducts produced thereby. The lower the amount of ester remaining assolvent for the oxidation product, after oxidation is completed, thehigher is the solidification point of the oxidation mixture and as aresult thereof, the more difficult is its transport and handling in theliquid or, respectively, pastelike state.

The amounts of phthalic acids which are formed during the oxidation ofthe xylenes are of no importance in carrying out the process accordingto the present invention since they are dissolved or, respectively,suspended in the toluic acid esters present in the oxidation mixture andacting as solvents. Such formed phthalic acids are also esterified andthus separated from the oxidation mixture, after oxidation is completed.

According to the present invention it is, of course, possible to use notonly the various isomer xylenes as such but also their mixtures witheach other. Likewise, the various isomer toluic acid esters can be usedas such or in mixture with each other. starting mixture employed, thereare obtained the various isomer phthalic acid esters, after oxidation iscompleted.

The phthalic acid esters obtained according to the present invention canbe separated from the other compounds obtained on oxidation either bydistillation or by crystallization. They are useful either asplasticizers or as starting material for polycondcnsation products, forinstance, for producing fibers, filaments, foils, films, bristles, andthe like plastic materials. Furthermore they are valuable additives toplastics, resins, and the like products.

The following examples serve to illustrate the present inventionwithout, however limiting the same thereto.

Example 1 A mixture of 1.2 kg. of 96% p-xylene and 3.6 kg. of puremethyl ester of p-toluic acid is filled into a pressure-resistantoxidation vessel of liters capacity which is provided with reflux coolerand water separator, 10 g. of the cobalt salt of the first runningsobtained on distillation of coconut oil fatty acids are added thereto asa catalyst. Air is introduced into the mixture at a temperature of 160C. and a pressure of 5 atmospheres. 1.08 cu. m. of air, measured atatmospheric pressure, are introduced per hour. Oxidation is continuedfor 8 hours. Thereafter, a sample is taken. it shows that a product withan acid number of 157 and a saponification number of 428 is obtained.The mixture contains about 2.5% of non-oxidized xylene. Oxidation iscontinued for 4 more hours. A sample shows that the resulting oxidationproduct has an acid number of 197 and a saponification number of 468 andthat all the xylene is oxidized. Oxidation is continued for 8 morehours. The acid number of the oxidation mixture, after hours totaloxidation time, has increased to 250. 5.2 kg. of oxidation mixture areobtained.

The mixture is esterified by heating with 20 liters of methanol in thepresence of 180 g. of 98% sulfuric acid at 65 C. for 30 hOlllS.' Aftercooling, the crystallized dimethyl ester of terephthalic acid isfiltered ofi, the filtrate Depending upon the i is freed of excessmethanol, and the residue is washed free of sulfuric acid. 2.1 kg. ofthe dimethyl ester of terephthalic acid with a saponification number of578, an acid number of 0.6, and a melting point of 140.5 C. are obtainedin this manner.

The residue consists of 3.6 kg. of the methyl ester of toluic acid ofthe saponification number of 402 and the acid number of 6. 1.2 kg. offresh xylene are added to said ester and the mixture is again oxidizedin the same manner as described hereinabove after addition of 5 g. ofthe same catalyst. After 18 hours, the oxidation mixture has an acidnumber of 288 and a saponification number of 495. The mixture is againesterified as described hereinabove and yields 2.75 kg. of the dimethylester of terephthalic acid.

The remaining methyl ester of p-toluic acid is made up to 4.8 kg. by theaddition of p-xylene and the mixture is again oxidized as describedhereinabove. Oxidation, esterification, separation of the phthalic acidester, and restoration of the initial proportion of xylene to toluicacid ester by the addition of xylene may be repeated many times.

Example 2 A mixture of 10 kg. of 96% p-xylene and 20 kg. of technicalgrade methyl ester of p-toluic acid are filled into a pressure-resistantoxidation vessel of a capacity of 60 liters which is provided withreflux condenser and water separator. 60 g. of the cobalt salt of thelower fatty acids obtained as first runnings on distillation of coconutoil fatty acids are added thereto as catalysts. 5.4 cu. m. of air areintroduced into the reaction mixture per hour at a pressure of 1.5 atm.and at a temperature of C. After 6 hours the oxidation mixture has anacid number of and a saponification number of 427 and still contains5.6% of non-oxidized xylene. After 10 hours no more non-oxidized xylenecan be determined. After a total oxidation duration of 18 hours, theacid number of the reaction mixture has increased to 305. 32.25 kg. ofoxidation mixture are obtained. The mixture is esterified by means ofmethanol under pressure. The esterification product is added to methanolin order to precipitate the dimethyl ester of terephthalic acid. 14.3kg. of dimethyl ester of terephthalic acid are filtered 01f. Its acidnumber is 1.2, its saponification number 576', and its melting point 140C. The remaining ester which consists mainly of the methyl ester ofp-toluic acid is made up to 30 kg. by the addition of p-xylene and theresulting mixture is again oxidized as described hereinabove.

It is, of course, also possible to employ, in place of p-xylene and ofthe methyl ester of p-toluic acid, equimolecular amounts of o-xylene orm-xylene and of the methyl esters of o-toluic acid or m-toluic acid.Like- Wise, a mixture of xyleneS, such as a commercial productcontaining, for instance, 66% of m-xylene, 33% of p-xylene, and theremainder being o-xylene and ethyl benzene, as well as mixtures of themethyl esters of mtoluic acid and p-toluic acid with small amounts ofotoluic acid may also be employed as starting material. The procedure isthe same as described hereinabove in the examples.

Esterification of the oxidation mixture to form the diesters of theresulting phthalic acid and the esters of' toluic acids is effected, forinstance, by heating the oxidation mixture with anhydrous methanol in anautoclave at a temperature between about 200 C. and about 250 C. for aperiod of 2 to 3 hours. Thereafter the autoclave is vented and the sameamount of anhydrous methanol is added thereto and heating to atemperature between about 200 C. and about 250 C. is continued for 2 to3 hours. In this manner, the methyl esters'of said toluic acids and thedimethyl esters of the phthalic acids are obtained with an acid numberas low as 10 and less.

Esterification of the oxidation mixture may also be efiected byrefluxing the oxidation mixture with an excess of methanol and with theaddition of hydrochloric acid or other conventional esterificationcatalysts whereby the water produced during esterification iscontinuously removed from the distillate while the methanol is returnedinto the reaction vessel.

In place of the catalyst used in the preceding examples, there may beemployed other catalysts such as oobaltous and manganous caproate,caprylate, caprinate, l'aurate and the like. The concentration of thecatalyst employed is comparatively low and is usually in the range ofabout 0.001% to about 0.1% of cobalt or manganese metal based on thetotal weight of the reaction mixture.

Of course, many changes and variations in the composition of thestarting material and in the proportion of xylene and the lower alkylesters of toluic acid, in the reaction conditions such as temperature,pressure, and duration, in the catalysts used, in the methods ofesterifying the oxidation mixture and of separating the resultingesters, in the manner in which the esterified oxidation mixture ispurified and the 'like may be made by those skilled in the art inaccordance with the principles set forth herein and the claims annexedhereto.

Although the methyl esters of toluic acids are especially suitable forthe oxidation process according to the present invention, it is alsopossible to use other esters of said acids, for instance, the esterswith ethanol, npropanol, isopropanol, n-butanol, isobutanol, tertiarybutanol. Likewise, esterification of the oxidation mixture, afteroxidation is completed, may be carried out with such other loweralkanols. The procedure is, in general, the same as that described inthe preceding examples.

I claim:

1. In a process of producing esters of phthalic acids from mixtures ofxylene and esters of toluic acids with monovalent alkanols having 1 to 4carbon atoms, the steps which comprise oxidizing a mixture of xylenesand esters of toluic acids with alkanols having 1 to 4 carbon atoms inthe proportion by weight between about 1:3 and, at the most, 1:1 in theliquid phase with an oxygen-containing gas at a temperature betweenabout 80 C. and about 250 C. in the presence of an oxidation catalystfor a prolonged period of time until the xylenes are substantiallycompletely oxidized and continuing oxidation of the resultingintermediate, sub stantially xylene-free oxidation mixture until as muchtoluic acid ester is oxidized as corresponds to the amounts of toluicacids formed on xylene oxidation.

2. The process according to claim 1, wherein oxidation is carried out ata temperature between about 140 C. and about 160 C.

3. The process according to claim 1, wherein oxidation is carried outunder pressure.

4. The process according to claim 1, wherein a mixture of xylenes andthe methyl esters of toluic acids is used as starting material.

5. The process according to dation catalyst is the cobalt having 6 to 12carbon atoms.

6. In a process of producing esters from mixture of xylenes and estersof monovalent alkanols having 1 to 4 carbon atoms,

claim 1, wherein the oxisalt of an alkanoic acid of phthalic acidstoluic acids with the steps which comprise oxidizing a mixture ofxylenes and esters of toluic acids with monovalent alkanols having 1 to4 carbon atoms in the proportion by weight between about 1:3 and, at themost, 1:1 with an oxygen-containing gas at a temperature between aboutC. and about 250 C. in the presence of an oxidation catalyst for aprolonged period of time until the xylenes are substantially completelyoxidized, thereafter continuing oxidation of the resulting oxidationmixture until an amount of toluic acid esters is oxidized whichcorresponds to the amount of toluic acids formed on xylene oxidation,esterifying the resulting oxidation mixture, separating and removing thephthalic acid esters formed during oxidation from the toluic acid esterspresent in the oxidation mixture, adding to said toluic aci esters anamount of xylenes sufiicient to restore the proportion by weight ofxylenes and esters of toluic acid to a proportion between about 1:3 and,at the most, 1:1, and repeating oxidation of the mixture of xylenes andesters of toluic acids.

7. The process according to claim 6, wherein oxidation is carried out ata temperature between about C. and about C.

8. The process according to claim 6, wherein oxidation is carried outunder pressure.

9. The process according to claim 6, wherein the catalyst is the cobaltsalt of an alkanoic acid having 6 to 12 carbon atoms.

10. The process according to claim 6, wherein the ester of toluic acidsis the methyl ester of said acids.

11. The process according to claim 6, wherein oxidation andesterification are carried out in continuous operation.

12. In a process of producing the dimethyl ester of terephthalic acid,the steps which comprise oxidizing a mixture of p-xylene and the methylester of p-toluic acid in the proportion by weight between about 1:3and, at the most, 1:1, at a temperature between about 140 C. and about160 C. with air under a pressure between about 1.5 atm. gauge and about5 atm. gauge with the addition of the cobaltous salt of the fatty acidsobtained as first runnings in the distillation of coconut oil fattyacids for a prolonged period of time until the xylene is substantiallycompletely oxidized, continuing oxidation of the resulting oxidationmixture until an amount of the methyl ester of p-toluic acidcorresponding to the amount of p-toluic acid formed on oxidation ofp-xylene, is oxidized, esterifying the reaction mixture by heating withmethanol in the presence of an esterification catalyst, cooling theesterification mixture, removing, by filtration, the crystallizeddimethyl ester of terephthalic acid, evaporating excess methanol fromthe filtrate, admixing to the evaporation residue substantiallyconsisting of the methyl ester of p-toluic acid, p-xylene in an amountsuflicient to restore the proportion by weight of p-xylene and themethyl ester of p-toluic acid to a proportion between about 1:3 and, atthe most, 1:1, and repeating oxidation of the resulting mixture ofp-xylene and the methyl ester of p-toluic acid.

References Cited in the file of this patent UNITED STATES PATENTS2,772,305 Levine et a1. Nov. 27, 1956

1. IN A PROCESS OF PRODUCING ESTERS OF PHTHALIC ACIDS FROM MIXTURES OFXYLENE AND ESTERS OF TOLUIC ACIDS WITH MONOVALENT ALKANOLS HAVING 1 TO 4CARBON ATOMS, THE STEPS WHICH COMPRISE OXIDIZING A MIXTURE OF XYLENESAND ESTERS OF TOLUIC ACIDS WITH ALKANOLS HAVING 1 TO 4 CARBON ATOMS INTHE PROPORTION BY WEIGHT BETWEEN ABOUT 1:3 AND, AT THE MOST, 1:1 IN THELIQUID PHASE WITH AN OXYGEN-CONTAINING GAS AT A TEMPERATURE BETWEENABOUT 80*C. AND ABOUT 250*C. IN THE PRESENCE OF AN OXIDATION CATALYSTFOR A PROLONGED PERIOD OF TIME UNTIL THE XYLENES ARE SUBSTANTIALLYCOMPLETELY OXIDIZED AND CONTINUING OXIDATION OF THE RESULTINGINTERMEDIATE, SUBSTANTIALLY XYLENE-FREE OXIDATION MIXTURE UNTIL AS MUCHTOLUIC ACID ESTER IS OXIDIZED AS CORRESPONDS TO THE AMOUNTS OF TOLUICACIDS FORMED ON XYLENE OXIDATION.